Of the various electrostatic printing techniques, the most familiar and widely utilized is xerography, wherein latent electrostatic images formed on a charge retentive surface, such as a roller, are developed by a toner material to render the images visible, the images being subsequently transferred to plain paper. This process is called an indirect process since the visible image first formed on an intermediate photoreceptor and then transferred to a paper surface.
Another form of electrostatic printing is known as direct electrostatic printing (DEP). Many of the methods used in DEP, such as particle charging, particle transport, and particle fusing are similar to those used xerography. However, this form of printing differs from xerography in that an electrical field is generated by electrical signals to cause toner particles to be deposited directly onto plain paper to form visible images without the need for those signals to be intermediately converted to another form of energy. It is this concept of simultaneous field imaging and particle transport to produce a visible image directly on plain paper that is novel to direct electrostatic printing.
U.S. Pat. No. 5,036,341 granted to Larson discloses a DEP printing device and a method to produce text and pictures with toner particles on an image receiving substrate directly from computer generated signals. The Larson patent discloses a method which positions a control electrode array, comprised of a latticework of individually controlled wires, between a back electrode and a rotating particle carrier. An image receiving substrate, such as paper, is then positioned between the back electrode and the control electrode array.
An electrostatic field on the back electrode attracts the toner particles from the surface of the particle carrier to create a particle stream toward the back electrode. The particle stream is modulated by voltage sources which apply an electrical potential to selected individual wires of the control electrode array to create electrical fields which permit or restrict transport of toner particles from the particle carrier. In effect, these electric fields "open" or "close" selected apertures in the control electrode array to the passage of toner particles by influencing the attractive force from the back electrode. The modulated stream of charged particles allowed to pass through selected apertures impinge upon a print-receiving medium interposed in the particle stream to provide line-by-line scan printing to form a visible image.
The control electrode array of the above mentioned patent is constructed of a lattice of individual wires arranged in rows and columns. However, a control electrode array may take on many designs. Generally, the array is a thin sheet-like element comprising a plurality of addressable control electrodes and corresponding voltage signal sources connected thereto for attracting the charged toner particles from the particle carrier to the receiving paper substrate by applying voltage signals to the control electrode array to create an electric field between the back electrode and the particle carrier to produce a visible image directly on plain paper. For example, the control electrode array may be constructed of a flexible, non-rigid material and overlaid with a printed circuit such that apertures in the material are arranged in rows and columns and are surrounded by electrodes. Regardless of the design or the material of construction, it is essential to minimize the gap distance between the control electrode array and the surface of the particle carrier to maintain a high print quality. However, this gap distance must not be so minimized as to allow contact between the charged particles on the carrier surface and the control electrode array.
The actual gap between the charged particles and the control electrode array can vary greatly from machine to machine as it is determined by a combination of independent factors such as manufacturing variations in the size and placement of the particle carrier and the control electrode array, as well as the thickness of the particle layer on the particle carrier.
In addition to minimizing the gap between the control electrode array and the particle carrier, it is also desirable to maintain a smooth uniform particle layer thickness on the particle carrier, and to preferably minimize the thickness of this layer to a single particle in depth. Typically, the diameter of an individual particle is on the order of 10 microns, with a particle layer on the particle carrier being approximately 30-40 microns thick.
Because the particle size is only on the order of 10 microns, even the slightest mechanical imperfections can result in a drastic degradation of print quality. For instance, the particle carrier is a rotating cylinder which is neither perfectly round nor perfectly smooth. This eccentricity, along with various surface imperfections on the carrier cylinder are only two of a number of potential irregularities which cause variations in the thickness of the particle layer. Further, the particles themselves may vary in their diameter and degree of sphericity. Thus, to accommodate all of these independent dimensional variations, the gap distance between the control electrode array and the particle carrier is typically increased as a safety factor to insure no contact between the two elements. Although this increased gap distance may insure that the variations in position and dimension do not cause the particle layer to contact the control electrode array surface, it is opposite to the desirability to minimize the gap distance to maintain high print quality.
In the prior art, scraper blades are used to restrict the thickness of the toner particle layer on the particle carrier. Excess particles are scraped from the carrier to reduce the layer thickness such that it was less than the gap distance and thus insured no contact between the control electrode array and the particles on the carrier.
The scraper blade and the control electrode array were both mounted to the printer frame in a fixed position. Typically the scraper blades were constructed of a non-flexible rigid material. In order to insure that the scraper blade did not contact the particle carrier, thus scraping off all of the particles, the scraper blade was offset at some minimum distance to accommodate variations in manufacturing and assembly of the printer. However, increasing the offset distance had the undesirable effect of also increasing the thickness of the particle layer on the carrier. Still further, the scraper blade could have surface imperfections along its scraping edge and might not be mounted perfectly parallel to the surface of the carrier cylinder.
Thus, to insure that there was no contact between the charged particles and the control electrode array, the gap between the particle carrier and the fixed control electrode array was necessarily increased to accommodate the maximum possible particle layer thickness.
As a result, this fixed clearance design with a rigid scraper resulted in an excessively thick particle layer and allowed for considerable variations in that thickness. This in turn required that the gap between the control electrode array and the particle carrier be fixed at a greater than desirable distance.
In an attempt to traverse the disadvantages associated with rigid blades, manufacturers introduced blades constructed of a flexible material such as rubber (see for example, U.S. Pat. No. 3,566,786). The flexible blades were better able to accommodate variations in manufacturing and assembly of the printer. However, because the blade was flexible, it did not consistently provide a uniform force across the length of and against the surface of the particle carrier. Thus, it was difficult to maintain a uniform thickness of particles to be conveyed to the control electrode array. Therefore, to insure that there was no contact between the particle layer and the fixed position control electrode array, the gap between the particle carrier and the control electrode array was again necessarily fixed at a greater than desirable distance to accommodate the maximum possible particle layer thickness.
Thus, there is a need for an improved means for maintaining a constant minimal gap between the control electrode array and the charged particle layer, while simultaneously insuring no contact between the particle layer and the surface of the control electrode array.